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I though that the radeonhd driver would have been merged into the ati driver...

nobody ever said it would. It's technically not feasible. Both drivers share some code, fixes and additions are often included in both etc, but there are some differences that cannot just be "merged". It may sound weird to a non-programmer, but right now it's less work to maintain both drivers than it'd be to merge them.

Both drivers might die in a few years anyway, being replaced by a generic Gallium3D ddx driver.

Both drivers might die in a few years anyway, being replaced by a generic Gallium3D ddx driver.

We don't even need that. If we switch to KMS, then DDX will responsible for:
1) Talking with KMS (radeon does it)
2) EXA and Xv acceleration (both driver do, almost the same way)

So with KMS none of driver-specific code is used. Then we really will not need radeonhd and radeon could be drastically cut from whole mode setting. However we will probably see both radeon and radeonhd supported for a lot of time still, for older users, not using KMS yet (which should be stable and feature-full in 2.6.33).

You'd be correct, it does. In fact, if the RadeonFeature page is up-to-date, the only thing the radeonhd driver has that radeon does not is HDMI audio for a couple more cards.

Plus better determining of minimum engine clock for LowPowerMode, better backlight support (RandR) and resolutions for panels (1600x900, 1600x1200, 1600x1200Scaled, 1920x1440, 1920x1080Scaled) and so on...
Radeon has support for PCI lanes reduction, TV-out and somehow DisplayPort.
That's probably all. If you decide to use KMS, all above doesn't matter, because you use use other code then (from kernel's drm).